1. Field of the Invention
The present invention relates to a pyroelectric type infrared radiation detecting device. More specifically, the present invention relates to an improved pyroelectric type infrared radiation detecting device including a pyroelectric pellet generating an electric charge as a function of a variation of an incidental infrared radiation and a base for supporting the pellet.
2. Description of the Prior Art
An infrared radiation detecting device employing a pellet having a pyroelectric effect has been proposed and put into practical use as a temperature sensor, an intruder alarm, and the like. As is well known, such a pyroelectric pellet is responsive to a variation in an incidental infrared radiation so as to generate on the surface thereof an electric charge, wherein the smaller the thermal emission coefficient of the pyroelectric pellet, the greater the quantity of generated electric charge. Heretofore, therefore, various structures have been proposed and implemented to suppress thermal dissipation from such a pyroelectric pellet.
FIG. 1 is a view showing one example of a structure of a conventional pyroelectric type infrared radiation detecting device which constitutes the background of the present invention. The conventional one shown in FIG. 1 comprises a pyroelectric pellet 1 having electrodes 2 and 3 on both main surfaces. The electrode 2 of the pyroelectric pellet 1 is connected to a mount base 4 by means of lead wires 6 made of gold, for example. In such a case the lead wires 6 also serve to support the pellet 1 at a position spaced apart from the mount base 4, as shown in FIG. 1. Connection of the electrode 2 of the pyroelectric pellet 1 and the lead wires 6 is made by means of an electrically conductive adhesive agent 7, such as a silver paste and the like. A lead wire 9 is also similarly connected to the electrode 3 of the pyroelectric pellet 1 by means of an electrically conductive adhesive agent 8. The lead wire 9 is connected to one of two terminals 10. The terminals 10 serve to withdraw an electric charge generated on the surface of the pyroelectric pellet 1. Although not shown in FIG. 1, it has been a most common practice that a chip integrally including a field effect transistor and operational amplifier and resistors therefor, is placed on the mount base 4, while the electrode 3 and the lead wire 9 are connected to the chip, so that the output from the chip is withdrawn through the terminals 10.
According to the FIG. 1 structure, it follows that an air layer exists between the pyroelectric pellet 1 and the mount base 4. Thermal conductivity of the air is extremely poor, so that the same serves as a heat insulation. Therefore, heat dissipated from the pyroelectric pellet 1 is small and accordingly the output from the pellet 1 and thus from the terminals 10 becomes large. However, such a structure as shown in FIG. 1 involves the problems to be discussed in the following. More specifically, since the thinner the thickness of the pyroelectric pellet, the higher the sensitivity thereof, it is preferred that the pellet be thinner than say 100 .mu.m. Nevertheless, since, with such a structure as shown in FIG. 1, it is necessary to individually handle such a thin pyroelectric pellet 1, precise work is required. Accordingly, productivity is very poor. Furthermore, since the pyroelectric pellet 1 is connected and supported only by the lead wires 6, the mechanical strength is weak and accordingly the reliability thereof is also poor.
Therefore, a pyroelectric type infrared radiation detecting device of such a structure as shown in FIG. 2 has also been proposed. The infrared radiation detecting device as shown in FIG. 2 comprises a pyroelectric pellet 1 fixed to a mount base 4 by means of a base 11 which is made of a material of extremely poor thermal conductivity such as quartz. The electrode 2 of the pyroelectric pellet 1 is fixed to the upper surface of the supporting base 11 by means of an electrically conductive adhesive agent 12 such as a silver paste. One end of the lead wire 6 is connected to a portion of the electrically conductive adhesive agent 12, while the other end of the lead wire 6 is connected to the terminal 10.
Since such a conventional device as shown in FIG. 2 can make thermal dissipation from the pyroelectric pellet 1 substantially equal to that of the FIG. 1 device, the mechanical strength can be increased and reliability can be enhanced, without adversely affecting the output of the pyroelectric pellet 1. In addition, since the pyroelectric pellet 1 can be lapped while fixed on the supporting base 11, the working efficiency can be enhanced and hence mass production is improved. However, with such a structure as shown in FIG. 2, further problems are involved. More specifically, the pyroelectric pellet 1 is extremely thin, as described above, and the electrodes 2 and 3 and the conductive adhesive agent layer 12 are also very thin, as thin as several tens .mu.m. Accordingly, it is extremely difficult to connect the lead wire 6 to the exposed side surface of the conductive adhesive agent layer 12 without causing short circuiting of the electrodes 2 and 3. Such difficulty in the connection of the lead wire 6 also causes degradation of the productivity thereof.
Therefore, one might think of substituting a conductive supporting base 13 made of a metallic or electrically conductive material for the supporting base 11 as shown in FIG. 2. Employment of such a conductive supporting base 12 makes it possible to withdraw an electric charge generated in the pyroelectric pellet 1 through the electrode 2, the electrically conductive ahesive layer 12 and the electrically conductive supporting base 13. However, simple employment of the conductive supporting base 13 involves the problems to be discussed in the following. More specifically, generally an electrically conductive material has good thermal conductivity and, accordingly, it follows that the pyroelectric pellet 1 is fixed to the electrically conductive supporting base 13 of a good thermal conductor through the electrode 2 and the electrically conductive adhesive agent layer 12, both of which have good thermal conductivity. Therefore, thermal dissipation from the pyroelectric pellet 1 becomes larger than that of the devices shown in FIGS. 1 and 2 and accordingly the output from the pyroelectric pellet 1 decreases. Thus, although such a device as shown in FIG. 3 can enhance productivity thereof, such structure adversely affects the device characteristics, such as a detection sensitivity.